This invention pertains to glass-reinforced polyester molding compositions and more particularly to those containing as additives mixtures of lactone polymers and vinyl chloride polymers.
Compositions based on unsaturated polyesters are finding increased use in the automotive industry as compositions from which can be molded fenders, dashboards, and other like component parts of automobiles. As a general rule, these compositions contain in addition to the unsaturated polyesters low-profile additives such as poly (vinyl acetate). A low-profile additive is a material designed to insure that there is no undesirable shrinkage by the composition to which the additive has been added as the composition is being molded into a thermoset article. In other words, low-profile additives have been added to unsaturated polyester compositions which can be molded into thermoset articles the surfaces of which truly reflect the surface characteristics of the mold.
It has been found, however, that although the use of low-profile additives, as described, does effect some degree of improvement in the anti-shrinkage characteristics of the unsaturated polyester compositions, the improvement is more than offset by other problems occasioned by these low-profile additives when the compositions to which they have been added are to be used in sheet-molding applications. Sheet-molding applications involve forming the compositions into sheet material, winding the sheeted material into rolls, allowing the sheeted compositions to thicken and thereafter, as the need arises, unwinding and molding the sheeted material into thermoset articles of desired shape.
A major advance in commercial thermosetting molding technology was the introduction several years ago of chemically thickened systems. Chemical thickening is always employed in sheet molding compounds ("SMC"), and is increasingly being used in bulk molding compounds ("BMC"). In such systems, an alkaline material such as magnesium oxide or magnesium hydroxide is added to, for example, an uncured polyester along with fillers, glass fiber, and other standard materials. The alkaline material interacts with residual acidity in the polyester to build viscosity. The thickened system is relatively tackfree and easy to handle, and the high viscosity carries the glass fiber reinforcement to the extremities of the mold during crosslinking of the system. Thus, the use of thickened systems has made a major contribution to the commercial expansion of polyester molding.
When it is desired to add a thickening agent to the composition of this invention, a wide variety are available and are known to those skilled in the art. These include the oxides and hydroxides of the metals of Group I, II and III of the Periodic Table. Illustrative examples of thickening agents include magnesium oxide, calcium oxide, calcium hydroxide, zinc oxide, barium oxide, magnesium hydroxide, and the like, including mixtures of the same. Thickening agents are normally employed in proportions of from about 0.1 to about 6 weight percent, based upon weight of polyester resin, plus monomer, plus low profile additive.
However, not all low-profile additives have afforded satisfactory results.
There has, therefore, been a continuing need for a polyester molding composition which provides better shrinkage control and smoother surfaces to the molded part that have heretofore been available with conventional preparations.